THERMAL AND ENERGY EVALUATION OF ROOF AND CEILING INSULATION FOR RESIDENTIAL BUILDING IN TROPICAL CLIMATE

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Thermal and Energy Evaluation of Roof and Ceiling Insulation for Residential Building in Tropical Climate

THERMAL AND ENERGY EVALUATION OF ROOF AND CEILINGINSULATION FOR RESIDENTIAL BUILDING IN TROPICALCLIMATE

Nor Zaini Zakaria1, 2, Peter Woods3 and Ahmad Ramly4

1Institute of Science, Universiti Teknologi MARA, Shah Alam, Malaysia2Faculty of Applied Sciences, Universiti Teknologi MARA, Shah Alam, Malaysia3Knowledge Management Center, Multimedia University, Cyberjaya, Malaysia4Faculty of Built Environment, University Malaya, Kuala Lumpu, [email protected]

ABSTRACT: This paper presents the findings on the thermal and energy evaluation of roofinsulation for residential buildings in tropical climate of Malaysia. The investigation was doneusing a whole-building thermal energy simulation software, Tas. A conventional conductiveinsulation material with thermal conductivity value of 0.039 Wm-1K-1 was used. This was separatelyinstalled at the roof pitch and laid above the ceiling. The evaluation was done by analysing theindoor temperature in the roof space and indoor living spaces. The optimum thickness of theinsulation for both installations was identified. The benefits of the roof and ceiling insulationwere appraised by the indoor temperature and cooling energy.Keywords: roof insulation, ceiling insulation, thermal energy performance, residential building

1. INTRODUCTION

Thermal insulation provides resistance to conductive, convective, or radiative heat transfer. Theinsulation materials can be grouped as conductive and reflective while the effects can be consideredas resistive, reflective, and capacitive. The conductive insulating material depends on the lowthermal conductivity (k-value) to provide the resistive effect by retarding heat transfer viaconduction and convection. The reflective type depends on the surface characteristics, i.e. lowsolar conductivity, high solar reflectance, and low emissivity to provide the reflective effect bypreventing heat transfer via radiation. The reflective effect is instantaneous while the resistiveeffect of conductive insulation materials provides quick response to intermittent heating and/orcooling of heavyweight structures. The resistive effect is most effective for heat transfer understeady-state conditions or when the rate of heat transfer is almost constant for long periods, suchas in heated or air-conditioned spaces. The impact is not very significant if the diurnal variation issmall where the temperature reduction is minimal. The insulation lowers the thermal transmittance(U-value), therefore similar heat transfer pattern could be obtained between insulated lightweightconstruction and non-insulated heavyweight construction (Kolokotroni and Young, 1990; Rogers,1964). Unlike the immediate resistive and the reflective effect, the capacitive effect is a functionof time due to the periodic heat transfer characterised by the time-lag factor and provides atemporal control of heat transfer.

For optimum performance, there should be a balance between the thermal mass and insulation.This could effectively moderate the daytime heat but impedes the night-time cooling (Hyde andDocherty, 1997). The benefit of thermal insulation is influenced by the envelope construction with

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respect to climatic conditions, hence it is prudent to analyse the cost effectiveness and identify theoptimum value (Bolatturk, 2008; Ozel and Pihtili, 2007; Dombayci, 2007).

Roof is the greatest receiver of solar radiation in tropical climate where heat is the prevailingproblem. Therefore, it must be designed to reject heat. Among the recommended design featuresare;

� thermal insulations above or beneath the ceiling� thermal insulations underneath the upper roof layer� aluminium foil beneath roof to reduce radiant heat transfer

In Malaysia, the installation of radiant barrier is a standard practice for roof in residential building,but not the thermal insulation. This study evaluates whole-building thermal and energy performancewith the supplementary application of conductive thermal insulation at the roof pitch and on theceiling.

2. METHODOLOGY

2.1 Objective

The aim of the study is to evaluate the thermal energy performance of roof insulation for residentialbuildings in tropical climate. Accordingly, the objectives are:

� to evaluate thermal and energy performance of roof insulation� to identify the optimum thickness for roof insulation� to evaluate thermal and energy performance of ceiling insulation� to identify the optimum thickness for ceiling insulation

2.2 Description of Building Model

The model is a real modern contemporary double-storey bungalow with built-up floor area of188 m2. Figure 1.1 and 1.2 display the perspective view of the house and floor plan of the upperfloor. Figure 1.3 shows the standard roof detailing with construction from outside to inside as;concrete roof tile, air space and radiant barrier. The materials and construction of the conventionalpractices.

Figure 1.2: Floor plan of upper floor

Bed 1 Bed 2

Bed 3

Familyroom

Figure 1.1: View of building

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2.3 Description of Roof Model

Three roof models used in this study are listed Table 1. These are base-case roof for reference(BASE), BASE with roof insulation thickness varied from 10 mm to 100 mm, and lastly BASEwith ceiling insulation varied from 10 mm to 100 mm. The roof insulation models are named asRIn01 (t = 10 mm), RIn02 (t = 20 mm),… to RIn10 (t = 100 mm). Similarly the ceiling insulationmodels are named as CIn01 (t = 10mm), CIn02 (t = 20 mm),… to CIn10 (t = 100 mm). For the RInmodel, the insulation is placed underneath the radiant barrier, while for the CIn model it is laiddirectly on the ceiling.

Table 1: Roof models

Construction Concrete tile Air Space Radiant barrier Roof insulation Ceiling insulationModel

1. BASE � = 0.9 50 mm 0.25 mm2. RIn � = 0.9 50 mm 0.25 mm 10 mm - 100 mm3. CIn � - mm 01mm 52.0mm 059.0 = 100 mm

2.4 Data Collection

A thermal energy computer simulation software, Tas (EDSL, 2008), was used to generate thenecessary data for the evaluation. It is a software package for dynamic whole-building thermal andenergy simulation. It comprises of four modules; model design, thermal, energy, and airflow. Therequired input data were building model, envelope constructions, internal heat gains, weather data,and design-day data. The Klang Valley Model Year Climate was used (Zain-Ahmed, A.. 2000) asthe climate data. A design-day is determined for the thermal performance simulation in naturalventilation. The worst day of 7th March was chosen as this would give the prediction for the worst-case scenario. A 365-day batch simulation was done for the annual energy performance.

Figure 1.3: Standard roof detailing

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Whole-building thermal energy simulations were run for each roof model. The evaluations weredone based on reduction of Tmax in family room on the upper floor. This space is selected to representthe impact on the living spaces underneath the ceiling. It was chosen it has the least exposedexternal wall. Therefore, the impact due to factors such as heat transfer through external walls andwindows were minimal. Thus, roof is the major source of external heat gain compared to the otherspaces. The impact in the roof space is also analysed for comparison to appraise the advantage ofthe ceiling. The analyses and evaluations were as follows:

1. Thermal performance

This is assessed by the highest temperature (Tmax), lowest temperature (Tmin), average temperature(Tmean) and temperature difference (Tdiff). The extreme conditions (Tmax and Tmin) are of interest toexamine the immediate impact, but the overall condition (Tmean) is also considered. Tdiff is used toquantify the advantages and disadvantages of the design options.

2. Energy performance

This is appraised by the annual sensible cooling load (CL) of the occupied spaces on the upper floor;the bedrooms and family room. A CL index (CLI) is introduced as an indicator for the cooling energydemands, defined as the CL per square meter of cooled floor area.

3. Identify optimum thickness

The optimum values are determined by the reduction of Tmax in family room on the upper floor.

3. RESULT AND ANALYSIS

Thermal and energy performances are analysed to evaluate the thermal impact and energyconsumption and savings with the use of the insulation at the roof pitch and on the ceiling. Theoptimum thickness of the insulation for the roof pitch and on the ceiling is identified.

3.1 Roof Insulation (RIn)

1. Thermal and energy performance

A summary for the bedrooms, family room roof space for RIn models is shown in Table 2. It showsfor each space, there appears to be a minimum Tmax that could be achieved in the natural ventilationmode. Additional insulation could not lower the temperature any further.

Due to minimal temperature variation of the models, Figures 2.1 and 2.2 show the impact of RInon roof space and family room for several selected models. This is calculated by temperaturedifference between the BASE model and RIn models (TBASE – TRIn). The charts clearly show theadvantage is more significant in roof space than in family room. Model RIn10 reduces the temperaturein roof space up to 6.7°C but in family room the reduction is up to only 0.3°C. The Tdiff in roof spaceshows that additional RIn lowers the daytime temperature but increases it at the night. However, theelevated night-time temperature is much less than the daytime reduction, thus could still be beneficial.Nevertheless, the minimal impact in family room necessitates the identification of an optimumthickness due to the cost of insulation material despite the reduction in CL.

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Table 2: Summary of thermal and energy performances for RIn models

Model Bed1 Bed2 Bed3 Family RSpace Annual CL CLI(kWh) (kWhm–2year–1)

RIn00 (BASE; Tmax (°C) 34.8 34.5 34.9 34.2 43.4 8556 143d = 0 mm) Tmin (°C) 26.7 27.7 27.8 27.2 28.0

Tmean (°C) 31.3 31.6 31.4 31.6 33.8

RIn01 Tmax (°C) 34.7 34.4 34.8 34.1 41.8 8459 141(d = 10 mm) Tmin (°C) 26.7 27.7 27.8 27.2 28.2

Tmean (°C) 31.3 31.6 31.4 31.5 33.4


Tmean (°C) 31.3 31.6 31.4 31.5 33.1


Tmean (°C) 31.3 31.5 31.4 31.5 32.9


Tmean (°C) 31.3 31.5 31.3 31.5 32.8


Tmean (°C) 31.3 31.5 31.3 31.4 32.6


Tmean (°C) 31.3 31.5 31.3 31.4 32.5


Tmean (°C) 31.3 31.5 31.3 31.4 32.4


Tmean (°C) 31.3 31.5 31.3 31.4 32.3


Tmean (°C) 31.3 31.5 31.3 31.4 32.3


Tmean (°C) 31.3 31.5 31.3 31.4 32.2

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2. Identification of optimum thickness

Graphs of daily Tmax and Tmean are plotted against the thickness of RIn as shown in Figures 2.3 and2.4 respectively. Both temperatures are considered to appraise the overall thermal impact. Thegraph of Tmax in Figure 2.3 shows the temperature gradient is either zero or 0.1°C. Based on the firstplateau of Tmax, the optimum thickness is 10 mm but the temperature is further reduced by 0.1°C for40 mm after which it remains constant for thickness up to 100 mm. Therefore, for the purposemaximising the effect of the insulation, 40 mm is identified as the optimum. However, the graph ofTmean in Figure 2.4 shows the 50 mm insulation further lowers the temperature by 0.02°C.Nevertheless, since Tmax remains unchanged, a further reduction of Tmean by a hundredth of a degreeCelsius is considered as not significant for human thermal sensation. Thus, it is concluded that 40mm is the optimum thickness for RIn. With respect to BASE model, Table 2 shows RIn04 with CLI138 kWhm–2year–1 reduces the annual CL by 280 kWh or 3.3%.

Figure 2.1: Tdiff in roof space between BASEand RIn models

Figure 2.2: Tdiff in family room between BASE andRIn models

INSULATION: Temperature difference in RSpace

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INSULATION: Temperature difference in Family

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Figure 2.3: Tmax in family room for RIn models Figure 2.4: Tmean in family room for RIn models

INSULATION: Tm ax vs d in Family

34.1034.1034.10

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34.0034.0034.0034.0034.0034.0034.00

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INSULATION: Tmean vs d in Family

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31.4431.43 31.4231.42

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d (mm)

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3.2 Ceiling insulation (CIn)

1. Thermal and energy performance

A summary for the bedrooms, family room roof space for CIn models is shown in Table 3. Similar tothe RIn models, it shows for each space, there is a minimum Tmax that could be achieved in thenatural ventilation mode. Additional insulation could not lower the temperature any further.

Table 3: Summary of thermal and energy performance for CIn models

Model Bed1 Bed2 Bed3 Family RSpace Annual CL CLI(kWh) (kWh–2year–1)

CIn00Tmax (°C) 34.8 34.5 34.9 34.2 43.4 8556 143 (BASE;t = 0 mm) Tmin (°C) 26.7 27.7 27.8 27.2 28.0

Tmean (°C) 31.3 31.6 31.4 31.6

CIn01 Tmax (°C) 34.8 34.5 34.9 34.2 43.8 8565 143(t = 10 mm) Tmin (°C) 26.7 27.7 27.8 27.2 27.6

Tmean (°C) 31.3 31.6 31.4 31.6


Tmean (°C) 31.4 31.6 31.4 31.6


Tmean (°C) 31.4 31.7 31.4 31.6

CIn04 Tmax (°C) 34.7 34.3 34.8 34.‘ 44.7 8531 142(t = 40 mm) Tmin (°C) 26.7 27.7 27.8 27.2 27.1

Tmean (°C) 31.4 31.7 31.5 31.6


Tmean (°C) 31.4 31.7 31.5 31.6


Tmean (°C) 31.4 31.7 31.5 31.6


Tmean (°C) 31.4 31.7 31.5 31.6


Tmean (°C) 31.4 31.7 31.5 31.6


Tmean (°C) 31.4 31.7 31.5 31.6


Tmean (°C) 31.4 31.7 31.5 31.6

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Similar to the thermal performance due to RIn, there are minimum values for Tmax and Tminfor each space. There is no reduction of the CLI for CIn of more than 20 mm. The CLI is142 kWhm–2year–1 for CIn of 20 mm and thicker. Using the same analyses for optimum RIn, agraph of daily Tmax shown in Figures 7.27 clearly shows the optimum CIn is 20 mm. The annualCL savings is less than 0.1%.

3.3 Summary

For all the models, the advantage of thermal insulation is more significant in the roof space.However, in residential builings in Malaysia, this space is not occupied. For the installation at theroof pitch, it reduces the Tmax in roof space up to 6.7°C. Nonetheless, the impact in the livingspaces underneath is insignificant with Tdiff of only 0.3°C. The optimum thickness for insulationinstalled at the roof pitch is 40 mm. and for ceiling insulation is 20 mm. The RIn04 model withCLI of CLI 138 kWhm–2year–1 reduces the annual CL by 280 kWh or 3.3%. The annual CL savingby CIn02 model with CLI 142 kWhm–2year–1 is trivial with less than 0.1%. A summary of the energyevaluation is presented in Figure 3.2 in terms of CL profiles.

4. CONCLUSION

The optimum thickness for insulation installed at the roof pitch is 40 mm. and for ceiling insulation is20 mm. Roof pitch insulation has insignificant thermal improvement but the energy savings of 3.3%is quite noteworthy. However, the ceiling insulation has no significant benefit for neither thermalcondition nor energy saving.

Figure 3.1: Tmax in family room for CIn models

Ceiling Insulation: Tmax vs d in Family

34.2 34.2

34.1 34.1 34.1 34.1 34.1 34.1 34.1 34.1 34.1

34.0

34.1

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Base 10 20 30 40 50 60 70 80 90 100d (mm)

Tmax

(C)

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ACKNOWLEDGEMENT

The authors would like to thank the Ministry of Higher Education for funding the study, and UniversitiTeknologi MARA and University Malaya for the support.

REFERENCES

Bolatturk, A. (2008). Optimum insulation thickness for building walls with respect to cooling and heatingdegree-hour in the warmest zone of Turkey. Building and Environment, 43, 1055-1064.

Dombayci, O. (2007). The environmental impact of optimum insulation thickness for external walls of buildings.Building and Environment, 42, 3855-3859.

EDSL (2008). http://www.edsl.net. Environmental Design Solution Ltd. Tas Building Simulation Software.Kolokotroni, M. & Young, A. N. (1990). Guidelines for bioclimatic housing design in Greece. Building and

Environment. 25 (4). 297-307.Pihtili, M. (2007). Optimum location and distribution of insulation layers on building walls with various

orientations. Building and Environment, 42, 3051-3059.Rogers, T. S. (1964). Thermal Design of Buildings. New York: John Wiley & Sons, Inc.Zain-Ahmed, A. (2000). Daylighting and Shading for Thermal Comfort in Malaysian Buildings. PhD thesis,

University of Hertfordshire. United Kingdom.

Figure 3.2: CL profiles for all models

CL profiles: demand, savings and CLI for each retrofit from BASE

100.098.9 98.0 97.3 96.7 96.3 95.9 95.6 95.4 95.2 95.0 95.893.5 92.0 91.0 90.3 89.7 89.2 88.8 88.5 88.3

100.199.9 99.8 99.7 99.6 99.6 99.6 99.5 99.5 99.5

0.0 1.1 2.0 2.7 3.3 3.7 4.1 4.4 4.6 4.8 5.0 4.26.5 8.0 9.0 9.7 10.3 10.8 11.2 11.5 11.7

0.1 0.2 0.3 0.4 0.4 0.4 0.5 0.5 0.5

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Documents

THERMAL AND ENERGY EVALUATION OF ROOF AND CEILING INSULATION FOR RESIDENTIAL BUILDING IN TROPICAL CLIMATE